This invention relates to an objective lens driving device suitable for use in an optical pickup unit of an optical disc drive and, in particular, to an objective lens holder for use in the objective lens driving device.
As well know in the art, an optical disc drive is a device for reading/writing information from/into an optical disc (CD, CD-ROM, CD-R/RW, DVD-ROM, DVD±R/RW, Blu-lay, HD DVD, or the like). In order to achieve reading/writing the information from/into the optical disc, the optical disc drive of this type comprises an optical pickup unit for irradiating a laser beam onto the optical disc and for detecting its reflected beam.
In the manner which is well known in the art, in DVD apparatuses, there is one in which a particular optical pickup unit is mounted in order to enable to record/reproduce data in/from both of the DVD and the CD. The particular optical pickup unit of the type is for carrying out recording or reproducing by selectively using two kinds of laser beams, namely, a laser beam having short wavelength (wavelength band of 650 nm) for the DVD and a laser beam having a long wavelength (wavelength band of 780 nm) for the CD. The particular optical pickup unit is called a two-wavelength handling optical pickup unit.
One of the two-wavelength handling optical pickup units of the type described comprises a first laser diode (LD) for emitting the laser beam (a first laser beam) having the short wavelength for the DVD and a second laser diode (LD) for emitting the laser beam (a second laser beam) having the long wavelength for the CD. Such a two-wavelength handling optical pickup unit is disclosed in Japanese Unexamined Patent Application Publication No. 2003-272220 or JP-A 2003-272220.
However, if the first laser diode and the second laser diode are formed as separate parts, it is inconvenient that the two-wavelength handling optical pickup unit comprises a lot of parts and is large-scale. In order to cope with such problems, a new laser diode comprising, as one part (one chip), the first laser diode and the second laser diode is developed and proposed, for example, in Japanese Unexamined Patent Application Publication No. 11-149652 or JP-A 11-149652. Such a new laser diode is called a one-chip type laser diode. It is possible to miniaturize the two-wavelength handling optical pickup unit by using the one-chip type laser diode.
However, inasmuch as the one-chip type laser diode has a first emission point for emitting the first laser beam and a second emission point for the second laser beam that are apart from each other by a predetermined distance of, for example, 100 μm, the first laser beam and the second laser beam are emitted in parallel with they apart from each other by the predetermined distance. Accordingly, various problems can arise when one of two laser beams apart from each other is irradiated on the optical disc. It is therefore preferable to guide the first laser beam and the second laser beam to the same optical axis by using any optical axis coinciding means.
A two-wavelength laser module solving such a problem is proposed, for example, in Japanese Unexamined PatentApplication Publication No. 2001-284740 or JP-A 2001-284740. The proposed two-wavelength laser module comprises a first laser source for emitting a first laser beam having a first wavelength, a second laser source for emitting a second laser beam having a second wavelength different from the first wavelength, and optical axis coinciding means for receiving one of the first laser beam and the second laser beam to emit a laser beam on the same optical axis. In the two-wavelength laser module, the first laser source, the second laser source, and the optical axis coinciding means are mounted in a package.
In addition, in the above-mentioned one-chip type laser diode, when the two laser beams apart from each other are irradiated on the optical disc, return beams reflected thereon (disc's reflected beams) are also reflected (returned) with optical axes of them deviated from each other. Accordingly, in this state as it is, it is impossible to receive the disc's reflected beams at one reception position in a photodetector.
An optical pickup unit solving this problem is also proposed, for example, in Japanese Unexamined PatentApplication Publication No. 2002-288870 or JP-A 2002-288870. The proposed optical pickup unit comprises a two-wavelength package laser diode, an optical system, and an optical axis combining element. The two-wavelength package laser diode emits first and second laser beams, which have first and second wavelengths different from each other, in parallel from first and second emission points apart from each other by a predetermined distance, respectively. The optical system is for guiding the first and the second laser beams to an optical disc and is for transmitting first and second return beams having deviated optical axes incident from the optical disc. The optical axis combining element guides the first and the second return beams transmitted through the optical system to the photodetector so that the deviated optical axes are coincided at one reception position of the photodetector.
At any rate, the optical disc drive of the type described develops a tendency to a thin type (a slim type or a ultra-slim type) so as to have a low height size. As a result, it is necessary to thin an optical pickup actuator which is a main portion of the optical pickup unit.
In general, an optical pickup unit comprises a laser beam source for emitting a laser beam and an optical system for guiding the emitted laser beam to an optical disc and for guiding its reflected beam to a photodetector. The optical system includes an objective lens disposed so as to face the optical disc. It is necessary for the objective lens used in the optical pickup unit to accurately control in position with respect to a focus direction along an optical axis and a track direction along a radial direction of the optical disc to thereby accurately focus the laser beam on a track of a recording surface of the rotating optical disc. These controls are called a focusing control and a tracking control, respectively. Further, following improvement in recording density, there have recently been increasing demands for removing or suppressing the influence caused by warping of the optical disc. In view of this, it is also necessary that the objective lens be subjected to a so-called tilting control.
The above-mentioned optical pickup actuator is a device for enabling the focusing control, the tracking control, and the tilting control. The optical pickup actuator is called an objective lens driving device. In the objective lens driving device, an objective lens holder holding the objective lens is elastically supported by a suspension member with respect to a damper base. The suspension member consists of a plurality of suspension wires disposed both sides of the damper base and the objective lens holder.
Now, the objective lens driving devices are classified into a so-called symmetry type and a so-called asymmetry type. The objective lens driving devices of the symmetry type are ones wherein coils and a magnetic circuit including magnets are symmetrically disposed with respect to the objective lens as a center. The objective lens driving devices of the asymmetry type are ones wherein the coils and the magnetic circuit including magnets are asymmetrically disposed with respect to the objective lens.
One of the objective lens driving devices of the symmetry type is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 2001-93177 or JP-A 2001-93177. According to the JP-A 2001-93177, the objective lens driving device of the symmetry type comprises an objective lens holder for holding an objective lens, a focusing coil wound around the objective lens holder, tracking coils affixed to the objective lens holder at outer sides in a tangential direction of an optical disc, and tilting coils affixed to the objective lens holder at both sides in a radial direction of the optical disc. These coils are partly located in gaps of the magnetic circuit. With this structure, the objective lens driving device of the symmetry type is capable of finely controlling a position and an inclination of the objective lens by controlling currents flowing through the respective coils. In addition, inasmuch as it is necessary to affix the tracking coils and the tilting coils to the sides of the objective lens holder, each of the tracking coils and the tilting coils comprises an air-core coil.
However, the conventional objective lens driving device of the symmetry type, as disclosed in the JP-A 2001-93177, is disadvantageous in that it takes a deal of time to manufacturing the objective lens driving device and it increases in cost. This is because it is necessary to affix the tracking coils and the tilting coils to the sides of the objective lens holder. In addition, the objective lens driving device is expensive because the air-core coil is very expensive compared with a winding coil. Furthermore, inasmuch as the conventional objective lens driving device of the symmetry type has a structure where the focusing coil is wound around the objective lens holder, the laser beam must be passed through under an lower surface of the objective lens holder and it is therefore difficult to thin the optical pickup unit.
On the other hand, one of the objective lens driving device of the asymmetry type is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 2001-273656 or JP-A 2001-273656. According to the JP-A 2001-273656, the objective lens driving device comprises coils printed on a printed board. By using the objective lens driving device of the asymmetry type, it is possible to expect to miniaturize (thin) the optical pickup unit.
However, the conventional objective lens driving device of the asymmetry type, as disclosed in the JP-A 2001-273656, is disadvantageous in that it is difficult to keep weight balance due to its asymmetry structure and undesired resonance such as rolling, pitching, or yawing of the object lens easily occurs on mass production. In addition, the conventional objective lens driving device of the asymmetry type is disadvantageous in that the coils printed on the printed board are expensive compared with winding coils. This is because a manufacturing process for printing the coils on the printed board is complicated. And, the conventional objective lens driving device of the asymmetry type comprises magnets magnetized in a manner of complicated multiple-poles. Therefore, the conventional objective lens driving device of the asymmetry type is expensive. By using such magnets, degradation of acceleration sensitivity, deterioration of a tilt characteristic, and so on are caused due to a neutral zone.